Steps to reproduce:
1. Create a new project
2. Create a parent canvas
3. Create a child canvas
4. Create a prefab out of the child canvas
5. Modify any of the prefab's components' values
6. Drag the prefab into the scene as a child of the parent canvas

Expected result: the original values for the rect transform are retained
Actual result: rect transform values get set to what they would be if the canvas did not have a parent

Reproduced on 2018.3.0a6(introduction of improved prefabs), 2018.3.0b12, 2019.1.0a11